Trace elements in the shoreline and seabed sediments of the southern Caspian Sea: investigation of contamination level, distribution, ecological and human health risks, and elemental partition coefficient

This study assesses the occurrence of trace elements (TEs) in sediments of the southern Caspian Sea. A total of 16 shoreline sediment samples and 15 seabed sediment samples along five coastal transects were studied. The mean concentration of TEs follows the order of Zn > V > Cr > Ni > Cu > Pb > Co > As > Sb > Mo > Cd. The TEs had an uneven, heterogeneous distribution within the shoreline and seabed sampling sites. This is due to that the study area comprises a large number of different pollution sources, also different sediment physicochemical characteristics. Levels of individual TEs within the seabed sediment transects were higher where their shoreline sites had higher concentrations, reflecting that the coastal sites play an important role in diffusing the contaminants towards the sea. The main anthropogenic source of TEs in this highly populated region, especially in the western part, is likely a large number of discharge points of greywater entering the sea. In addition, dominant fishing industry, tourism, intense agriculture, and textile and paper industry, as well as several other commercial activities, contribute significantly to the overall loading of TEs. Based on the statistical analyses, the organic matter and mud fraction had a strong explanatory value for the spatial variation of Cu, while oxyhydroxides of Fe and Mn had good explanatory factors to govern the spatial variation of other TEs. Pb and Zn had a relatively high partition coefficient (Kd), reflecting the affinity of these elements to be sorbed to the sediment phase. Cd and Sb had lower Kd, tending to remain in the aqueous phase. Geochemical indices indicated high enrichment of Cd, Sb, Zn, and Pb at a number of sampling sites, reflecting potential local sources of contamination. The Sisangan recreational area was identified as the most contaminated site. From a public health perspective, the non-carcinogenic risk of TEs was significant only at this site. The carcinogenic risks of Pb(II) and As(III) in adults, and Pb(II), Cd(II), and As(III) in children, were tolerable.

Microplastic fibers in the gut of highly consumed fish species from the southern Caspian Sea

This study assesses the frequency, distribution, characteristics, and chemical composition of microplastics (MPs) in the gut of highly consumed fish species, namely leaping mullet (Chelon saliens), common carp (Cyprinus carpioi), and Caspian kutum (Rutilus caspicus), in the southern Caspian Sea biome. Fibers are found to be the only shape of MPs. Black MPs and polystyrene, polypropylene, and polyethylene terephthalate polymers are dominant. MP frequency is highest in leaping mullet's gut, while kutum specimens exhibited the lowest MP frequency, reflecting that leaping mullet is a neritic species and thus highly exposed to MP influx in shallow coastal water, while the other species are benthopelagic. The estimated condition index reflected a significant difference between the species, implying that MPs may pose adverse health impacts on leaping mullet and common carp, with no undesirable effect on Caspian kutum. No significant relationship exists between biological parameters and the MP frequency in the fish gut.

Hydrogeochemical and ecological risk assessments of trace elements in the coastal surface water of the southern Caspian Sea

This study investigates the occurrence, distribution, and potential ecological risk of trace elements (TEs) along with the hydrogeochemical characteristics of coastal surface water collected from the southern Caspian coasts in the Mazandaran province of Iran. Eighteen coastal water sites were sampled and analyzed using inductively coupled plasma-mass spectrometry and ion chromatography to determine concentrations of TEs and major ions, respectively. Mean concentrations (µg/l) of TEs in the water followed the order: Al (154.3) > Fe (73.6) > Zn (67.8) > Mn (29.9) > Cu (5.7) > Mo (3.7) > Cd (2.8) > Se (2.3) > V (1.9) > Co = As (1.8) > Sb (1.2) > Pb (0.6). TEs displayed high variations within samples, reflecting many sources that control their concentrations in the coastal water. Most TEs displayed elevated concentrations in the east and west of the study area. The Na-Cl water type in the majority of investigated sites indicates excess alkaline elements and strong acid anions relative to alkaline earth cations and weak acid anions. Considering the saturation states of minerals, carbonate and evaporite minerals are oversaturated and unsaturated in surface water, respectively. Compositional interrelations between aqueous species showed that reverse cation exchange may have occurred. The excess SO₄^2- content, derived from irrigation return flow and domestic greywater, probably plays a crucial role in the mobilization and transport of Zn and Pb by binding to sulfate ligands and forming aqueous complexes. Ecological risk assessment of TEs revealed that water in the majority of sites is safe in terms of As, Se, Pb, and Cd content, and unsuitable with respect to Zn and Cu. Acute and chronic toxicities of Cu and Zn are reported in several sites, thus coastal water cannot be used for fishery and protecting "nature reserve" purposes. However, industrial activity and tourism are safe to be conducted in most coastal water sites.

Microplastic particles in sediments and waters, south of Caspian Sea: Frequency, distribution, characteristics, and chemical composition

This study assesses the occurrence of microplastics (MPs) in coastal and sea surface sediments, as well as water samples, collected from the coastal region of the southern Caspian Sea, Mazandaran province, Iran. A total of 32 sediment and 10 water samples were studied. The mean concentration of MPs was 15 units kg^-1 in the sediments and 710 units m^-3 in the coastal water. Fibers constituted by far the dominant MPs in both media, accounting for 97% of the MPs in both sediment and water samples. The MPs were mainly black in color. The dominant size of MP particles in sediment samples was between 250 and 500 μm, while the fraction >1000 μm dominated in the water samples. Polyethylene terephthalate (PET), polystyrene (PS), and nylon (NYL) were the main polymers and/or copolymers composing MPs in both sediment and water samples. The MP particles had a relatively smooth surface morphology, although signs of weathering were observed. The number of MP particles in sediment and water samples showed a general decrease from west to east in the study area. This may be reflecting the spreading of MP loading from the outlets of Sefidrud, Tonekabon, Chalus, the major rivers entering the Caspian Sea just west of the study area, and the overall decrease in the spatial distribution of touristic and fishery activity. The main sources of MP particles could be local emissions from a large number of domestic wastewater effluents and urban surface runoff due to high population density, and industrial and fishing activities in this region. This study indicated that MP particles, based on their characteristics and chemical composition, are circulated between coastal waters, and shore and sea surface sediments of the Caspian Sea, leading to their uneven distribution in the different depths. To the best of our knowledge, this is the first work studying the distribution of MP particles in sea surface sediments and also the most comprehensive on MPs in shoreline sediments and coastal waters in the southern Caspian Sea.